Method for dispersing aqueous based drilling fluid for drilling subterranean boreholes

ABSTRACT

An inhibitive water-based polymer mud system and method for using the system in drilling and in stabilizing wellbores for use in water sensitive formations as an alternative to oil-based muds or water-based muds comprising ferro-chrome lignosulfonates. The system comprises a fresh water or salt water base thinned or dispersed with a sulfonated acrylic copolymer having a hybrid/graft lignosulfonate multipolymer structure containing carboxylate and sulfonate functional groups with synthetic polymer side chains covalently linked to a base lignosulfonate material, having a molecular weight in the range of about 1,000 to about 15,000, and having a high anionic charge density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to controlling the rheology and/or theviscosity of water based mud systems. More particularly, the presentinvention relates to methods and compositions for dispersing aqueousbased fluids used in well drilling and other well operations insubterranean formations, especially subterranean formations containingoil and/or gas. This invention also relates to a drilling fluid thinnerand/or dispersant having improved temperature stability, dispersingproperties and “solids contamination” tolerance.

2. Description of Relevant Art

A drilling fluid or mud is a specially designed fluid that is circulatedthrough a wellbore as the wellbore is being drilled to facilitate thedrilling operation. The various functions of a drilling fluid includeremoving drill cuttings or solids from the wellbore, cooling andlubricating the drill bit, aiding in support of the drill pipe and drillbit, and providing a hydrostatic head to maintain the integrity of thewellbore walls and prevent well blowouts. Specific drilling fluidsystems are selected to optimize a drilling operation in accordance withthe characteristics of a particular geological formation.

For a drilling fluid to perform its functions, it must have certaindesirable physical properties. The fluid must have a viscosity that isreadily pumpable and easily circulated by pumping at pressuresordinarily employed in drilling operations, without undue pressuredifferentials. The fluid must be sufficiently thixotropic to suspend thecuttings in the borehole when fluid circulation stops. The fluid mustrelease cuttings from the suspension when agitating in the settlingpits. It should preferably form a thin impervious filter cake on theborehole wall to prevent loss of liquid from the drilling fluid byfiltration into the formations. Such a filter cake effectively seals theborehole wall to inhibit any tendencies of sloughing, heaving or cave-inof rock into the borehole. The composition of the fluid should alsopreferably be such that cuttings formed during drilling the borehole canbe suspended, assimilated or dissolved in the fluid without affectingphysical properties of the drilling fluid.

Most drilling fluids used for drilling in the oil and gas industry arewater-based muds. Such muds typically comprise an aqueous base, eitherof fresh water or brine, and agents or additives for suspension, weightor density, oil-wetting, fluid loss or filtration control, and rheologycontrol. Controlling the viscosity of water based muds or mud systemshas traditionally been done with lignosulfonate deflocculants and/orthinners. Such low molecular weight, heavily sulfonated polymers arebelieved to aid in coating clay edges in the subterranean formation witha lasting or effectively permanent negative charge. Some alkalinematerial, such as, for example, caustic soda or potash, is typicallyadded to achieve a pH range from about 9.5 to about 10. This pHenvironment is believed to aid the solubility and activation of theportion(s) of the lignosulfonate molecules that interact with the clay.These portions are believed to be the carboxylate and phenolate groupson the lignosulfonate.

Lignosulfonates are obtained from byproducts of the spent acid processused to separate cellulose from wood in the pulp industry. The pulpindustry has begun to turn away from the spent acid process in recentyears in favor of another process that does not have a lignosulfonatebyproduct. Consequently, the drilling fluid industry has begun effortsto find a substitute for lignosulfonates in drilling fluids. Also,increasingly, there is an interest in and need for deflocculants and/orthinners that can work effectively at lower pH ranges of about 8 toabout 8.5, in freshwater and saltwater based muds, and at highertemperatures ranging upwards to about 450° F., while also beingenvironmentally compatible.

SUMMARY OF THE INVENTION

The present invention provides improved methods of drilling wellbores insubterranean formations employing water-based muds and compositions foruse in such methods. As used herein, the term “drilling” or “drillingwellbores” shall be understood in the broader sense of drilling orwellbore operations, to include running casing and cementing as well asdrilling, unless specifically indicated otherwise.

The drilling fluids of the invention comprise an aqueous base and awater soluble thinner/dispersant comprising an acrylic copolymer of lowmolecular weight (in the range of about 1,000 to about 15,000) with highanionic charge density. In one embodiment, the thinner/dispersantcomprises a sulfonated polymer having a molecular weight of 5000 to10,000. In another embodiment, the thinner/dispersant comprises asulfonated polymer having a molecular weight of 4,000 to 8,000. Thepolymer chemistry for both of these embodiments is that of ahybrid/graft lignosulfonate multipolymer, containing both carboxylateand sulfonate functional groups, where synthetic polymer side chains aregrown covalently off of a base lignosulfonate material.

This thinner/dispersant for use in the invention has the flexibility ofutility with, and solubility in, a fresh water drilling fluid base, aswell as in a salt water (brine) drilling fluid base, and is effectiveeven at a near neutral pH of about 8.0 to about 8.5, while still beingeffective at a higher pH, up to about 10.5. Moreover, thethinner/dispersant may be used in solid or liquid form.

Drilling fluids of the invention provide an advantage over fluidsemploying prior art lignosulfonate thinners in that the fluids of theinvention maintain satisfactory rheology for drilling at temperatures ashigh as 400° F. to 450° F. while also being useful at lowertemperatures, including temperatures approaching as low as 40° F.Moreover, the thinner/dispersant used in the present invention does notcontain chromium, commonly used with prior art lignosulfonate thinners,and thus the present invention is more environmentally friendly orcompatible than such prior art.

Methods of the invention include a method of drilling a wellbore in asubterranean formation employing an aqueous based drilling fluidcomprising the thinner/dispersant discussed above and a method ofthinning or dispersing a water-based drilling fluid using suchthinner/dispersant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing the yield point versus dispersantconcentration of two different thinners/dispersants of the inventionwith a ferro-chrome, lignosulfonate thinner, in a gypsum/salt water, 22lb/bbl AQUAGEL, containing 66 lb/bbl Rev Dust, hot rolled at 150° F.,with a pH in the range of 9.2 to 10.

FIG. 2 is a graph comparing the yield point versus concentration of twodifferent thinners/dispersants of the invention with a ferro-chrome,lignosulfonate thinner, in fresh water containing 66 lb/bbl Rev Dust and22 lb/bbl Bentonite Slurry, hot rolled at 300° F.

FIG. 3 is a graph comparing the yield point versus pH of two differentthinners/dispersants of the invention with a ferro-chrome lignosulfonatethinner in a concentration of 5 lb/bbl in fresh water containing 66lb/bbl Rev Dust and 22 lb/bbl Bentonite Slurry, hot rolled at 300° F.

FIG. 4 is a graph comparing the gel strength versus pH of two differentthinners/dispersants of the invention with a ferro-chrome lignosulfonatethinner and a chrome-free lignosulfonate thinner in a concentration of 5lb/bbl in a fresh water drilling fluid, hot rolled at 300° F.

FIG. 5 is a bar graph correlating average molecular weight ofthinners/dispersants to the yield point of bentonite slurry treated with5 lb/bbl thinners/dispersants, hot rolled at 300° F.

FIG. 6 is a bar graph comparing the yield point of samples of a 16 ppgwater-based drilling fluid having no thinner, having 5 lb/bbl (100%active) ferro-chrome lignosulfonate thinner, and having two different 5lb/bbl (100% active) thinners/dispersants of the invention, hot rolledat 300° F.

FIG. 7 is a bar graph comparing the yield point of samples of a 14 ppgwater-based drilling fluid having no thinner, having 5 lb/bbl (100%active) ferro-chrome lignosulfonate thinner, and having two different 5lb/bbl (100% active) thinners/dispersants of the invention, hot rolledat 300° F., and hot rolled at 400° F.

FIG. 8 is a bar graph comparing the relative API filtrate of 14 lb/galfresh water-based drilling fluid treated with either 5 lb/bbl (100%active) ferro-chrome lignosulfonate thinner or with two different 5lb/bbl (100% active) thinners/dispersants of the invention.

FIG. 9 is a bar graph comparing the filtration control properties of athinner/dispersant of the invention with a ferro-chrome lignosulfonate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In methods of the present invention, drilling of shales and offshorehydratable formations may be conducted with an aqueous-based drillingfluid which affords rheology control and maintains wellbore stabilityeven though the wellbore penetrates smectites, illites, and mixed layerclays and even though the wellbore temperatures exceed about 350° F. andapproach temperatures of about 400° F. or higher.

The present invention provides thinners/dispersants that impart thinningto water based drilling fluids or mud systems comparable to or betterthan prior art lignosulfonate deflocculants and/or thinners whileeffecting such thinning at lower concentration, and less caustic pH,namely about 8.0 to 8.5, than with prior art lignosulfonatedeflocculants and/or thinners. Moreover, such thinners/deflocculants ofor for use in the invention have the advantage of containing notransition group elements (i.e., heavy metals such as chromium, andcadmium) and are believed to be more environmentally friendly than priorart deflocculants/thinners containing such heavy metals. Further, thethinners/dispersants are effective at thinning water based muds ordispersing solid particles in water based muds at high temperatures andover a relatively broad pH range (from about 8.0 to about 10.5), and aretolerant of contaminants such as cement, anhydrite and sodium, as wellas drill solids.

The thinners/dispersants of or for use in the invention comprise anacrylic copolymer of low molecular weight with high anionic chargedensity. In one embodiment, the thinner/dispersant comprises asulfonated polymer having a molecular weight of 5000 to 10,000. Inanother embodiment, the thinner/dispersant comprises a sulfonatedpolymer having a molecular weight of 4,000 to 8,000. The polymerchemistry for both of these embodiments is that of a hybrid/graftlignosulfonate multipolymer, containing both carboxylate and sulfonatefunctional groups, where synthetic polymer side chains are growncovalently off of a base lignosulfonate material.

The thinners/dispersants for use in the invention have a high thinningefficiency and can mitigate the flocculating effect of electrolyte(salt) in water based fluids even at temperatures as high as 400° F. orhigher. The thinners/dispersants effect thinning and/or deflocculationin saltwater based fluids and in fresh water based fluids and arebelieved useful and readily soluble in any water based mud suitable foruse in drilling or well operations in a subterranean formation,particularly for the discovery and/or recovery of oil and/or gas. Suchmuds should not contain chromium (or other similar heavy metals) andmost preferably will have a pH of about 8.0 to about 8.5, although thethinners/dispersants of or for use in the invention will providethinning and/or deflocculation over a pH range of about 8.0 to about10.5.

In one embodiment, the drilling fluid of the present invention comprisesthe above thinner or dispersant in an amount that thins the particulardrilling fluid as needed for drilling. The drilling fluid maintainsrheology and filtration control with a significantly lower quantity,i.e., lower concentration, of thinner/dispersant of the invention thanis typically used when prior art thinners, dispersants or deflocculantsare used.

In a method of the present invention of drilling a wellbore in asubterranean formation, a water based drilling fluid of the inventioncontaining a thinner or dispersant of the invention is used. In oneembodiment, the thinner or dispersant is provided with a pH environmentof about 8.0 to about 8.5. In another embodiment, the pH may be as highas 10.5 or otherwise in the range of 8.0 to 10.5. In one embodiment, thedrilling fluid has a brine base, and in another embodiment, the drillingfluid has a fresh water base. The subterranean formation may have atemperature as low as 40° F., as high as 400° F., or some temperature inbetween.

The following experiments and examples are illustrative of theadvantages of the invention.

EXPERIMENTAL

Drilling fluid samples were prepared according to test procedures in API13J, known to persons of ordinary skill in the art, and incorporatedherein by reference. Generally, 350 ml drilling fluid samples wereprepared and sheared on a multi-mixer for 60 minutes and then rolled inan oven at the test temperature. Bentonite slurry was mixed according tothe Quality Assurance Laboratory Standard Test Procedure for ThinningEfficiency of Fe/Cr Lignosulfonates (STP 17.01.002.01), incorporatedherein by reference. A Fann 35A from Fann Instruments was used for therheology measurements. A pH meter model 420A+ from Thermo Orion was usedfor the pH determinations. A Zetasizer Nano Series from MalvernInstruments was used to determine the zeta potentials of thethinners/dispersants. The zeta potential of the thinners/dispersants wasdetermined by preparing known concentrations of the thinners/dispersantsin ethanolamine buffer at pH 9.4. The zeta potential was measured at 25Vand 25° C. using the Zetasizer instrumentation.

Thinning efficiency of the thinners/dispersants was calculated asfollows:

${{Thinning}\mspace{14mu}{efficiency}} = {\left( \frac{\left( {{{YP}\mspace{14mu}{Control}\mspace{14mu}{Mud}} - {{YP}\mspace{14mu}{Test}\mspace{14mu}{Mud}}} \right)}{\left( {{{YP}\mspace{14mu}{Control}\mspace{14mu}{Mud}} - {{YP}\mspace{14mu}{QUIK}\text{-}{THIN}\; ®}} \right.} \right) \times 100}$where the Control Mud, the Test Mud and the QUIK-THIN® thinner mud allhad the same composition except that the Control Mud had no thinneradded thereto, the Test Mud had a test thinner added thereto, and theQUIK-THIN® thinner mud had QUIK-THIN® thinner added thereto. QUIK-THIN®thinner is a ferro-chrome lignosulfonate commercially available fromHalliburton Energy Services, Inc. in Houston, Tex. and Duncan, Okla.,that helps to control rheological and filtration properties ofwater-based drilling fluids and can be used to maintain dispersedwater-based drilling fluids. YP is an abbreviation for yield point.

A 221b/bbl Bentonite slurry, sheared in Gypsum/Salt water, was treatedseparately with QUIK-THIN® thinner, and two thinners/dispersants of theinvention at various concentrations and then rolled at 150° F. for 16hours for initial screening tests. Tables 1 and 2 depict the rheologicalproperties measured with a Farm 35A for three lb/bbl and five lb/bbltreatments of the thinners/dispersants. The corresponding ThinningEfficiency (TE) was calculated from the above equation. As can be seenin Tables 1 and 2, not only did the thinners/dispersants of theinvention compare well with the ferro-chrome lignosulfonate thinner, butthe Thinning Efficiency of the thinners/dispersants increased with adecrease in the amount of thinner used relative to the ferro-lignosulfonate thinner. This is particularly significant becauseferro-lignosulfonates, and particularly QUICK-THIN® thinner, are leadingprior art dispersants for clay particles in salt water.

TABLE 1 Thinning Efficiency of 5 lb/bbl Dispersants on Salt Water Mudafter Hot-Rolling at 150° F. for 16 hours Thinner/ Thinner/ Ferro-ChromeDispersant Dispersant Control Lignosulfonate A of Invention B ofInvention pH 7.06 9.43 9.55 9.84 600 rpm 60 45 46 57 300 rpm 50 32 30 37 6 rpm 26 14 11 12  3 rpm 25 13 10 11 10 s gel, 21 14 16 18 lb/100 ft210 m gel, 23 19 13 35 lb/100 ft2 PV, cP 10 13 16 20 YP, 40 19 14 17lb/100 ft2 TE, % 100 114 110

TABLE 2 Thinning Efficiency of 3 lb/bbl Dispersants on Salt Water Mudafter Hot-Rolling at 150° F. for 16 hours Thinner/ Thinner/ Ferro-ChromeDispersant Dispersant Control Lignosulfonate A of Invention B ofInvention pH 7.06 9.13 9.21 9.83 600 rpm 60 55 50 53 300 rpm 50 42 32 37 6 rpm 27 20 19 17  3 rpm 26 19 17 16 10 s gel, 21 20 18 19 lb/100 ft210 m gel, 23 55 16 25 lb/100 ft2 PV, cP 10 13 18 16 YP, 40 29 14 21lb/100 ft2 TE, % 100 236 173

The trend of the effect of the thinners/dispersants on the yield point(YP) of the salt water bentonite slurry is shown in FIG. 1. Under theconditions tested, the two thinners/dispersants of the invention showedan optimum thinning concentration at about 3 lb/bbl. This is anindication of the effectiveness of the thinners/dispersants of theinvention in preventing the reduction of the repulsive forces by theelectrolytes and allowing the clay particles to be dispersed at lowerconcentrations of the thinners than with ferro-chrome lignosulfonates.

A similar test was performed on the bentonite slurry in fresh water.Again, the results were very satisfactory for the thinners/dispersantsof the invention. In comparison to the ferro-chrome lignosulfonate, thethinners/dispersants of the invention were effective in thinning theslurry at lower concentrations. FIG. 2 shows the yield point of thethinners/dispersants at various concentrations after rolling the sampleat 300° F. for 16 hours.

In order to act as good deflocculants in either fresh water or saltwater, lignosulfonate thinners require some caustic soda to bring the pHof the fluid in the range of 8.5-10. At this pH level it is believed,without wishing to be limited by theory, that most of the acidicfunctionalities are deprotonated to increase the anionic charge density.Thus, the pH effect on the thinners/dispersants of the invention wasevaluated relative to the lignosulfonate thinners.

Bentonite slurry containing 66 lb/bbl of drilled solid material in freshwater was prepared and samples were treated with 5 lb/bbl of QUIK-THIN®thinner, ENVIRO-THIN™ thinner or a thinner/dispersant of the inventionand then rolled for 16 hours at 300° F. Thereafter, each of the treatedfluid samples was tested for yield point and gel strengths while the pHwas varied. The results of the yield point and gel strength were plottedagainst the pH as shown in FIGS. 3 and 4. The yield point values of thesamples with the thinners/dispersants of the invention were all lowerthan the yield point values of the samples with the lignosulfonatethinners for the pH range tested. The results show that the thinningeffect of lignosulfonate is more pH dependent than the thinning effectwith the thinners/dispersants of the invention.

Since the thinners adsorb on the edges of the clay particles to maintainan electric double layer and fortify the repulsive forces, the size ofthe double layer that results will depend on the anionic charge densityof the thinner. The zeta potential will not decrease because theelectrolyte tolerance capacity will increase with increase in chargedensity.

The zeta potentials and the electrophoretic mobility of thethinners/dispersants of the invention were measured and the results areshown in Table 3 in comparison with the ferro-chrome lignosulfonatethinner. The zeta potentials of the thinners/dispersants of theinvention compared favorably with the ferro-chrome lignosulfonatethinner, which is indicative of why the thinners/dispersants performedwell in the salt water bentonite slurry, and zeta potential can becorrelated to charge density.

TABLE 3 Zeta Potential Measurement Zeta Potential Electrophoretic TestMaterial (mV) Mobility (um*cm/vs) Thinner/Dispersant 19 −2 A ofInvention Thinner/Dispersant 18 −2 B of Invention Ferro-Chrome 20 −2Lignosulfonate

The lignosulfonate that results from spent sulfite liquor containspolymers having different degrees of sulfonation and varying molecularweights ranging from 1,000 to 20,000. It is much easier to control themolecular weight of a synthetic polymer, such as the thinner/dispersantof the invention. To investigate the effect of molecular weight on theefficiency of thinners/dispersants of the invention, four morethinners/dispersants substantially similar in structure and function toThinner/Dispersant A of the invention (and to Thinner/Dispersant B ofthe invention, which itself is substantially similar in structure andfunction to Thinner/Dispersant A), except in molecular weight, wereprepared, added to bentonite slurry, and yield point measured. Themolecular weight of each polymer sample was measured in the Zetasizer(using an aqueous solution of each thinner/dispersant). FIG. 5 shows thecorrelation of the average weight of these polymers and the yield pointof the bentonite slurry, treated with 5 lb/bbl of each polymer, androlled for 16 hours at 300° F. This data shows that the deflocculatingpower of the thinners/dispersants has a direct relationship with themolecular weight of the thinners/dispersants. The yield point of theslurry increased with increasing molecular weight of thethinners/dispersants, for these examples. The decrease in the thinningeffect with increasing molecular weight is due to decrease in the chargedensity if the molecular weight is increased without increasing thecorresponding anionic moieties.

Evaluations of thinning efficiencies of thinners/dispersants asdiscussed above is not enough to determine whether thethinner/dispersant will be an efficient and effective thinner when usedin a drilling fluid or that the drilling fluid will be effective withsuch thinner. Thus, various laboratory water-based drilling fluid (mud)samples were mixed and used to evaluate the efficiency of thethinners/dispersants of the invention in use in drilling fluids and theoverall effectiveness of the drilling fluids with thesethinners/dispersants.

One laboratory barrel of each mud sample was mixed on the Hamilton BeachMulti-mixer for 60 minutes according to the formulations contained inthe corresponding Tables 4-9 below. The thinners (in liquid form) wereadded to the mud samples at a 100% active level relative to the thinners(in solid form). The samples were pressurized in the mud cells androlled for 16 hours at the temperatures indicated in the tables for eachmud. In addition to QUIK-THIN® thinner, a ferro-chrome lignosulfonate,the following trademarked products are used in the Tables below:ALDACIDE® G biocide; AQUAGEL® viscosifier, a finely ground,premium-grade Wyoming sodium bentonite which meets the AmericanPetroleum Institute (API) Specification 13A, section 9 requirement;AQUAGEL GOLD SEAL® viscosifier, a 200 mesh, dry-powdered, premium,high-yielding Wyoming sodium bentonite containing no polymer additivesor chemical treatments of any kind; BARAZAN® D PLUS viscosifier, apremium quality, powdered Xanthan gum polymer; BAROID® weightingmaterial, a ground Barite that meets API Specification 13A section 7requirement; ENVIRO-THIN™ thinner, a modified iron lignosulfonate thatcontains no chrome or other heavy metals; and FILTER-CHEK™ filtrationcontrol agent. All of these trademarked products are available fromHalliburton Energy Services, Inc. in Houston, Tex. and Duncan, Okla.

TABLE 4 16.0 lb/gal Freshwater Mud Formulation with 35 lb/bbl Rev DustMud Formulations and Properties Sample Mark A B C D Freshwater, bbl0.687 AQUAGEL 4 GOLD SEAL ® viscosifier, lb AQUAGEL ® 4 viscosifier, lbFILTER-CHEK ™ 3 filtration control agent, lb Rev Dust, lb 35 BAROID ®weighting 385 material, lb ALDACIDE ® 0.2 G biocide, lb Dry causticsoda, lb 0.35 Thinner/Dispersant A of — 5 — — Invention, lbThinner/Dispersant B of — — 5 — Invention, lb QUIK-THIN ® — — — 5thinner, lb 12.5N Liquid caustic — 0.7 1.3 0.6 soda, total ml Rolled@300° F., hr 16 16 16 16 Stirred, min 15 15 15 15 Temperature, ° F. 120Plastic viscosity, cP 27 27 27 24 Yield point, lb/100 ft² 61 −4 −5 20 10Sec gel, lb/100 ft² 65 1 1 17 10 Min gel, lb/100 ft² 215 2 1 65 pHbefore rolling/ 9.06/8.21 9.55/8.45 9.54/8.47 9.55/7.96 pH after rollingFann 35 dial readings 600 rpm 115 48 49 68 300 rpm 88 21 22 44 200 rpm75 17 17 35 100 rpm 65 8 8 76  6 rpm 56 1 1 16  3 rpm 55 1 1 15 pHadjusted to 9.60 9.40 9.33 Plastic viscosity, cP 27 29 28 Yield point,lb/100 ft2 −3 −3 11 10 Sec gel, lb/100 ft2 2 1 13 10 Min gel, lb/100 ft23 2 72 Fann 35 dial readings 600 rpm 51 55 68 300 rpm 24 26 39 200 rpm18 18 29 100 rpm 9 10 18  6 rpm 2 1 8  3 rpm 1 1 7

TABLE 5 14.0 lb/gal Freshwater Mud Formulation with 60 lb/bbl Rev DustMud Formulations and Properties at 300° F. Sample Mark A B C DFreshwater, bbl 0.743 AQUAGEL 8 GOLD SEAL ® viscosifier, lb AQUAGEL ® 8viscosifier, lb FILTER-CHEK ™ 3.0 filtration control agent, lb Rev Dust,lb 60 BAROID ® weighting 249 agent, lb ALDACIDE ® 0.2 G biocide, lb Drycaustic soda, lb 0.35 Thinner/Dispersant A of — 5 — — Invention, lbThinner/Dispersant B of — — 5 — Invention, lb QUIK-THIN ® — — — 5thinner, lb 12.5N Liquid caustic — 0.7 1.3 0.6 soda, total ml Rolled@300° F., hr 16 16 16 16 Stirred, min 15 15 15 15 Temperature, ° F. 120Plastic viscosity, cP 32 56 45 43 Yield point, lb/100 ft² 111 7 9 102 10Sec gel, lb/100 ft² 98 9 3 65 10 Min gel, lb/100 ft² — 25 25 118 pHbefore rolling/ 9.06/7.98 9.55/8.43 9.44/8.68 9.85/8.16 pH after rollingFann 35 dial readings 600 rpm 175 119 99 188 300 rpm 143 63 54 145 200rpm 132 43 35 123 100 rpm 123 23 18 100  6 rpm 115 4 2 74  3 rpm 112 3 170 pH adjusted to — 9.55 9.46 9.31 Plastic viscosity, cP — 84 56 56Yield point, lb/100 ft2 — 20 6 75 10 Sec gel, lb/100 ft2 — 35 3 65 10Min gel, lb/100 ft2 — — 43 114 API, mL/30 min — 2.8 3.5 6.3 Fann 35 dialreadings 600 rpm — 188 118 187 300 rpm — 104 62 131 200 rpm — 73 42 119100 rpm — 40 23 85  6 rpm — 6 3 57  3 rpm — 4 2 55

TABLE 6 14.0 lb/gal Freshwater Mud Formulation with 60 lb/bbl Rev DustMud Formulations and Properties at 400° F. Sample Mark A B C DFreshwater, bbl 0.743 AQUAGEL 8 GOLD SEAL ® viscosifier, lb AQUAGEL ® 8viscosifier, lb FILTER-CHEK ™ 3.0 filtration control agent, lb Rev Dust,lb 60 BAROID ® weighting 249 agent, lb ALDACIDE ® 0.2 G biocide, lb Drycaustic soda, lb 0.35 Thinner/Disperant A of — 5 — — Invention, lbThinner/Dispersant B of — — 5 — Invention, lb QUICK-THIN ® — — — 5thinner, lb 12.5N Liquid caustic — 0.7 1.3 0.6 soda, total ml Rolled@400° F., hr 16 16 16 16 Stirred, min 15 15 15 15 Temperature, ° F. 120Plastic viscosity, cP 48 66 66 44 Yield point, lb/100 ft² 164 15 7 99 10Sec gel, lb/100 ft² 122 4 4 — 10 Min gel, lb/100 ft² 154 84 63 — pHbefore rolling/ 9.02/7.05 9.55/8.43 9.54/8.36 9.55/8.06 pH after rollingFann 35 dial readings 600 rpm 260 147 139 187 300 rpm 212 81 73 143 200rpm 188 54 48 117 100 rpm 182 29 24 105  6 rpm 141 3 3 85  3 rpm 140 2 284 pH adjusted to 9.41 9.35 9.33 Plastic viscosity, cP 71 85 76 Yieldpoint, lb/100 ft2 5 8 93 10 Sec gel, lb/100 ft2 4 4 113 10 Min gel,lb/100 ft2 73 53 262 Fann 35 dial readings 600 rpm 147 178 245 300 rpm76 93 169 200 rpm 50 65 145 100 rpm 26 33 127  6 rpm 3 3 117  3 rpm 2 2115

TABLE 7 13.0 lb/gal Freshwater Mud Formulation with 35 lb/bbl Rev DustMud Formulations and Properties Sample Mark A B C D Freshwater, bbl0.791 AQUAGEL ® 8 viscosifier, lb BARAZAN ® D PLUS 0.25 viscosifierFILTER-CHEK ™ 3 filtration control agent, lb Rev Dust, lb 35 BAROID ®weighting 219 agent, lb ALDACIDE ® 0.2 biocide G, lb Dry caustic soda,lb 0.35 Thinner/Dispersant A of 3 — — — Invention, lb Thinner/DispersantB of — 3 — — Invention, lb QUIK-THIN ® — — 3 — thinner, lb ENVIRO-THIN ™3 thinner, lb 12.5N Liquid caustic 1.3 1.2 1.3 1.4 soda, total ml Rolled@300° F., hr 16 16 16 16 Stirred, min 15 15 15 15 Temperature, ° F. 120Plastic viscosity, cP 18 19 19 19 Yield point, lb/100 ft² 4 5 10 12 10Sec gel, lb/100 ft² 2 3 4 8 10 Min gel, lb/100 ft² 4 4 10 30 pH beforerolling/ 9.60/7.08 9.55/7.29 9.54/8.30 9.55/7.91 pH after rolling Fann35 dial readings 600 rpm 40 43 48 50 300 rpm 22 24 29 31 200 rpm 17 1923 25 100 rpm 10 11 15 17  6 rpm 2 3 5 8  3 rpm 1 4 4 7

TABLE 8 14.0 lb/gal Freshwater Mud Formulation with 60 lb/bbl Rev DustMud Formulations and Properties Sample Mark A B C Freshwater, bbl 0.743AQUAGEL 8 GOLD SEAL ® viscosifier, lb AQUAGEL ® 8 viscosifier, lbFILTER-CHEK ™ 1.5 filtration control agent, lb Rev Dust, lb 60 BAROID ®weighting 249 agent, lb ALDACIDE ® 0.2 G biocide, lb Dry caustic soda,lb 0.35 Thinner/Dispersant A of 2.5 — — Invention, lb Thinner/DispersantB of 2.5 — Invention, lb QUIK-THIN ® — 2.5 thinner, lb 12.5N Liquidcaustic 0.9 1.3 0.6 soda, total ml Rolled @300° F., hr 16 16 16 Stirred,min 10 10 10 Plastic viscosity, cP 56 71 49 Yield point, lb/100 ft² 1210 105 10 Sec gel, lb/100 ft² 4 4 125 10 Min gel, lb/100 ft² 71 65 250API, mL 6.8 3.3 9.0 HPHT, mL @500 psi 19 16 26 pH before rolling/7.61/9.88 7.40/10.0 8.16/9.35 pH after rolling Fann 35 Reading 600 rpm124 152 203 300 rpm 68 81 154 200 rpm 46 59 138 100 rpm 25 32 130  6 rpm4 5 112  3 rpm 3 4 109

TABLE 9 14.0 lb/gal Freshwater Mud Formulation with 60 lb/bbl Rev DustMud Formulations and Properties Sample Mark A B Freshwater, bbl 0.7430.743 AQUAGEL 8 8 GOLD SEAL ® viscosifier, lb AQUAGEL ® 8 8 viscosifier,lb FILTER-CHEK ™ 1.5 3 filtration control agent, lb Rev Dust, lb 60 60BAROID weighting 249 249 agent, ® lb ALDACIDE ® 0.2 0.2 biocide G, lbDry caustic soda, lb 0.35 0.35 Thinner/Dispersant B of 2.5 — Invention,lb QUIK-THIN ® — 5 thinner, lb 12.5N Liquid caustic 0.9 0.6 soda, totalml Rolled @300° F., hr 16 16 Stirred, min 60 60 Plastic viscosity, cP 4837 Yield point, lb/100 ft² 10 52 10 Sec gel, lb/100 ft² 3 53 10 Min gel,lb/100 ft² 48 146 API, mL 7 8 HPHT, mL, 200° F. @500 psi 26 26 pH beforerolling/ 7.3/9.3 8.2/9.3 pH after rolling Fann 35 Reading 600 rpm 106126 300 rpm 58 89 200 rpm 40 79 100 rpm 22 56  6 rpm 3 40  3 rpm 2 38

FIG. 6 depicts the yield point of the 16 lb/gal mud samples treated with5 lb/bbl of a thinner/dispersant of the invention in comparison with aferro-chrome lignosulfonate thinner at 300° F. The thinning effect ofall of the thinners varied with pH but the thinners/dispersants of theinvention reduced the yield point lower than the ferro-chromelignosulfonate thinner (see Table 4).

The thinning efficiency of the thinners/dispersants was evaluated on a14 lb/gal mud having 10% drill solids (Rev Dust) and a highconcentration of bentonite. All mud samples were treated with a 5 lb/bblthinner/dispersant at 100% active level. The mud samples were rolled at300° F. and 400° F. for 16 hours. The yield point values of the mudsamples are shown in FIG. 7 (see also Tables 5 and 6). The yield pointof the mud containing the lignosulfonate thinner decreased compared tothe untreated base mud, but the yield point values of the mud sampleswith the thinners/dispersants of the invention were much lower than theyield point values of the mud containing the lignosulfonate thinner. Theincrease in temperature to 400° F. resulted in significant increase inthe yield point values of the lignosulfonate mud samples while the yieldpoint values of muds with the thinners/dispersants of the inventionminimally increased. This observation demonstrates the advantages of thepresent invention at high temperatures above 350° F.

The effect of thinners/dispersants of the invention on the filtrationcontrol of the fluid systems was also evaluated and compared to theeffect of lignosulfonate thinners on such systems. Four 14 lb/gal and 13lb/gal mud samples were made and treated with 5 lb/bbl of the thinners,and rolled at 300° F. for 16 hours. The pH of the fluids was adjustedwith 50% sodium hydroxide solution to 9.3-9.8. The results obtained fromthe API filtration testing are depicted in FIG. 8 for the sample'sfiltrate after 30 minutes. The thinners/dispersants of the invention hadfiltrate values about 50% lower than the ferro-chrome lignosulfonatethinner (see Tables 7 and 8). This observation demonstrates anotheradvantage of the present invention.

Further tests were conducted to determine the lowest amount ofThinner/Dispersant B of the Invention that would give filtration controlresults comparable to the ferro-chrome lignosulfonate thinner. A 14lb/gal mud was mixed and treated with different concentrations ofThinner/Dispersant B and the ferro-chrome lignosulfonate thinner, rolledat 300° F., and tested for both API and HPHT (high pressure hightemperature) filtration control. At half of the concentration of theferro-chrome lignosulfonate thinner, the Thinner/Dispersant B of theInvention would produce the same amount of thinning effect as theferro-chrome lignosulfonate. Further, only 50% of the Thinner/DispersantB of the invention was needed to produce the same amount offiltrate—provide the same or comparable filtration control, as theferro-chrome lignosulfonate. These comparative filtration results aredepicted in FIG. 9 and Table 9.

The above tests demonstrate that thinner/dispersants of the presentinvention lower the viscosity and gel strength of thick aqueous-baseddrilling fluids as well as or better than the best lignosulfonatethinners and such drilling fluids of the present invention have highdeflocculating power and are more salt tolerant and stable at hightemperatures than comparable fluids thinned with lignosulfonatethinners.

As indicated above, the advantages of the methods of the invention maybe obtained by employing a drilling fluid of the invention, includingthe thinner/deflocculant of the invention, in drilling operations. Thedrilling operations—whether drilling a vertical or directional orhorizontal borehole, conducting a sweep, or running casing andcementing—may be conducted as known to those skilled in the art withother drilling fluids. That is, a drilling fluid of the invention isprepared or obtained and circulated through a wellbore as the wellboreis being drilled (or swept or cemented and cased) to facilitate thedrilling operation. The drilling fluid removes drill cuttings from thewellbore, cools and lubricates the drill bit, aids in support of thedrill pipe and drill bit, and provides a hydrostatic head to maintainthe integrity of the wellbore walls and prevent well blowouts. Thespecific formulation of the drilling fluid in accordance with thepresent invention is optimized for the particular drilling operation andfor the particular subterranean formation characteristics and conditions(such as temperatures). For example, the fluid is weighted asappropriate for the formation pressures and thinned as appropriate forthe formation temperatures.

The foregoing description of the invention is intended to be adescription of preferred embodiments. Various changes in the details ofthe described fluids and methods of use can be made without departingfrom the intended scope of this invention as defined by the appendedclaims.

What is claimed is:
 1. A method of thinning an aqueous based drillingfluid for use in drilling a borehole in a subterranean formation,comprising adding to the drilling fluid an additive comprising anacrylic copolymer comprising a hybrid/graft lignosulfonate multipolymer,containing carboxylate and sulfonate functional groups, with syntheticpolymer side chains covalently linked to a base lignosulfonate material,and having a molecular weight in the range of 1,000 to 15,000 and a highanionic charge density as indicated by a zeta potential of about 18 to19 mV.
 2. The method of claim 1 wherein the copolymer has a molecularweight of 5000 to 10,000.
 3. The method of claim 1 wherein the copolymerhas a molecular weight of 4,000 to 8,000.
 4. The method of claim 1wherein the aqueous base is brine or saltwater.
 5. The method of claim 1wherein the aqueous base is fresh water.
 6. The method of claim 1wherein the fluid contains no chromium or other heavy metals.
 7. Themethod of claim 1 wherein the additive also provides filtration control.8. The method of claim 1 wherein the additive provides rheology controlat temperatures in the range of 350° F. to 400° F.
 9. A method fordrilling a borehole in a subterranean formation having water-sensitiveformations and temperatures greater than 350° F., said methodcomprising: providing an aqueous based drilling fluid thinned with anadditive comprising a sulfonated acrylic copolymer having a hybrid/graftlignosulfonate multipolymer structure containing carboxylate andsulfonate functional groups with synthetic polymer side chainscovalently linked to a base lignosulfonate material, wherein thecopolymer has a molecular weight in the range of 4000 to 10,000, and hasa high anionic charge density as indicated by a zeta potential of about18 to 19 mV; and drilling with the drilling fluid in the subterraneanformation.
 10. The method of claim 9 wherein said drilling fluid has apH in the range of 8.0 to 10.5 and the copolymer enhances therheological stability of the drilling fluid in the presence of fluidcontaminants.
 11. The method of claim 9 wherein said drilling comprisesdrilling through at least one producing zone in said formation.
 12. Themethod of claim 9 wherein said drilling comprises stabilizing thewellbore.
 13. New. The method of claim 9 wherein said thinned drillingfluid is stable at temperatures as low as 40° F. and as high as 400° to450° F.